JPH10256865A - Surface acoustic wave filter device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To secure the satisfactory out-band attenuation value by placing successively the parallel capacitance elements and then parallel inductance elements from the input/output terminal of a surface acoustic wave filter ele ment. SOLUTION: An input matching circuit 14a and an output matching circuit 14b are connected to an input terminal 13a and an output terminal 13b of a surface acoustic wave filter element 12 placed on a printed board 11. The circuits 14a and 14b secure the impedance matching between the element 12 and a transmission line 15 set on the board 11 and consists of the parallel capacitance elements 16 and the parallel inductance elements 17. The elements 16 are placed at the input/output terminal side of the element 12, and the elements 17 are placed at the position away from the elements 16 when viewed from the element 12. Then both elements 16 and 17 are electrically connected to each other.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a surface acoustic wave filter device. More specifically, the present invention relates to a surface acoustic wave filter device exhibiting improved out-of-band attenuation.

[0002]

2. Description of the Related Art A surface acoustic wave filter device was initially used as an IF filter for a TV, but has recently been actively used as an IF filter for various communication devices, mobile bodies, and mobile phones. It has become. In addition, the frequency to be used has been extended to a range exceeding 100 MHz, and so-called higher frequency has been promoted.

[0003] The types of filters used in these methods include a band-pass filter, a low-pass filter, a high-pass filter, and the like, which allows only an electric signal having a specific frequency component to pass therethrough, and an unnecessary signal having other frequency components. It has the function of blocking unwanted signals (and vice versa).
The frequency divisions are referred to as in-band (or in-passband) and out-of-band (or out-of-passband), respectively, of the filter. One of the important figures of merit for a filter is to increase the difference between the amount of signal passing through the band (the amount of passing) and the amount of signal blocked outside the band (the amount of attenuation).
In order to increase the difference, the filter elements are cascaded in multiple stages, the grounding method of the filter is improved, or the like, that is, the out-of-band attenuation is increased.

[0004]

By the way, 100 MHz
In the high frequency band exceeding, even if the filter elements are cascaded in multiple stages as described above or the grounding method is improved, the surface acoustic wave filter device in which the surface acoustic wave filter element is mounted on the printed circuit board is not enough. There was a problem that a large amount of out-of-band attenuation could not be obtained.

[0005] For example, PHS (Personal Handy Phone), which is one of the remarkable spread of mobile phone systems at present.
System) A surface acoustic wave filter device is used for an IF filter used in a terminal. The center frequency of this filter uses the 250MHz frequency band.
The attenuation (out-of-band attenuation) at a frequency distant from the center frequency by ± 21.5 MHz is required to be 60 dB or more. However, in a surface acoustic wave filter device in which the surface acoustic wave filter element for IF, which is a 250 MHz band and a high frequency band, is mounted on a printed circuit board, an out-of-band attenuation amount of about 50 dB or less can be obtained. there were.

Accordingly, an object of the present invention is to provide a surface acoustic wave filter device which exhibits a sufficient out-of-band attenuation even when a surface acoustic wave filter element of 100 MHz or higher, that is, a high frequency band is mounted on a printed circuit board. And

[0007]

The above object is achieved by the inventions described below. (Configuration 1) A surface acoustic wave filter device comprising: a surface acoustic wave filter element; and an impedance matching circuit connected to an input end and / or an output end of the surface acoustic wave filter element. The device is provided on a substrate on which a transmission line is formed, the impedance matching circuit includes at least one parallel inductance element and at least one parallel capacitance element, and from an input / output end of the surface acoustic wave filter element, The parallel capacitance element and the parallel inductance element are arranged and electrically connected in this order.

(Structure 2) In the surface acoustic wave filter device according to Structure 1, the parallel inductance element and the parallel capacitance element are arranged and electrically connected to only one side of the transmission line.

(Structure 3) In the surface acoustic wave filter device according to Structure 1, the parallel inductance element and the parallel capacitance element are arranged and electrically connected on both sides of a transmission line, respectively. And

(Structure 4) In the surface acoustic wave filter device according to any one of structures 1 to 3, the operating frequency of the surface acoustic wave filter device is 100 MHz or more.

(Structure 5) In the surface acoustic wave filter device according to any one of structures 1 to 4, the parallel inductance element is a chip inductor, and the parallel capacitance element is a chip capacitor. I do.

(Operation) Among the surface acoustic wave filter element, the inductance element, and the capacitance element constituting the surface acoustic wave filter device, an inductance element is considered to emit an electric field or a magnetic field, that is, an electromagnetic field outside the element. . Incidentally, an inductance element, for example, a chip inductor has a structure in which a winding is formed around a drum core, and a generated magnetic field leaks out of the element. Although shielded chip inductors have been commercialized to reduce the magnetic field leaking from the element, the magnetic field leaking outside the element cannot be eliminated. The same applies to an inductance element formed of, for example, a spiral conductive film. That is, it is considered that the magnetic field generated from the inductance element affects the capacitance element or the surface acoustic wave filter element. Also,
Since the transmission line formed on the printed circuit board also emits an electromagnetic field, it is considered that these electromagnetic fields influence each other in a complicated manner.

Therefore, although the cause is not clear, the surface acoustic wave filter device of the present invention can obtain the intended sufficient amount of out-of-band attenuation due to the complicated intertwining of the above-mentioned mutual effects of the electromagnetic fields. It is presumed that it is.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a plan view showing an embodiment of the present invention. Here, the surface acoustic wave filter element 12 installed on the printed circuit board 11 has a frequency of 250 MHz.
The one designed for the belt was used. An input matching circuit 14a and an output matching circuit 14b are connected to the input terminal 13a and the output terminal 13b of the surface acoustic wave filter element 12. The matching circuits 14a and 14b are provided in the surface acoustic wave filter element 1
2 is a circuit for matching impedance between the transmission line 2 and the transmission line 15 on the printed circuit board. The matching circuits 14a, 1
4b is composed of a parallel capacitance element 16 and a parallel inductance element 17.

In this embodiment, the parallel capacitance element 1
A chip capacitor of 9 pF was used for 6, and a chip inductor of 27 nH was used for the parallel inductance element 17. In these elements, a parallel capacitance element 16 is arranged on the input / output terminal side of the surface acoustic wave filter element 12, and a parallel inductance element 17 is arranged at a position distant from the parallel capacitance element 16 when viewed from the surface acoustic wave filter element 12. Connected to.

When the electrical characteristics (frequency characteristics) of the surface acoustic wave filter device of the present invention were measured, the attenuation at a frequency apart from the center frequency by ± 21.5 MHz was 66 dB. That is, it was found that the required value of 60 dB or more was sufficiently satisfied.

FIG. 2 shows an embodiment in which either the parallel capacitance 26 or the parallel inductance 27 in FIG. 1 is arranged on the opposite side of the transmission line 25. When the electrical characteristics of this surface acoustic wave filter device were measured, the attenuation at a frequency apart from the center frequency by ± 21.5 MHz was 65 dB. In this case as well, it was found that the required value of 60 dB or more was sufficiently satisfied.

That is, in the surface acoustic wave filter device of the present invention in which the parallel inductance element is disposed at a position apart from the parallel capacitance element with the surface acoustic wave filter element as a reference position, a sufficient out-of-band attenuation can be obtained. This has been proven.

FIG. 3 is produced for comparison with the present invention. Here, the surface acoustic wave filter element 3
2 used the same device as in FIG. Further, the same elements as those used in FIG. 1 were similarly used for the parallel capacitance element 36 and the parallel inductance element 37. The difference between FIG. 1 and FIG. 3 is that the arrangement of the parallel capacitance element and the parallel inductance element constituting the impedance matching circuit is different. That is, the surface acoustic wave filter element 32
The parallel inductance element 37 was installed on the side closer to and the parallel capacitance element 36 was installed on the far side. When the electrical characteristics of the surface acoustic wave filter device having this configuration were measured, only a 53 dB attenuation was obtained at a frequency ± 21.5 MHz away from the center frequency, and the required value of 60 dB or more was not satisfied.

FIG. 4, like FIG. 3, is made for comparison with the present invention. Here, the same elements as those used in FIG. 1 are used for the surface acoustic wave filter element 42, the parallel capacitance element 46, and the parallel inductance element 47. The arrangement of the parallel capacitance element 46 and the parallel inductance element 47 is such that the transmission line 45 is located at the same distance from the surface acoustic wave filter element 42.
Were arranged. When the electrical characteristics of the surface acoustic wave filter device having this configuration were measured, it was found that ± 21.
At a frequency separated by 5 MHz, only an attenuation of 48 dB was obtained, and also in this case, the required value of 60 dB or more was not satisfied.

Further, surface acoustic wave filter devices having the respective structures shown in FIGS. 1 to 4 described above using a surface acoustic wave filter element having a center frequency of 110 MHz are also manufactured.
The electrical characteristics were measured, and the same results as above were obtained.

[0022]

As described above with reference to the embodiments, according to the present invention, by arranging the parallel capacitance element and the parallel inductance element in this order from the input / output end of the surface acoustic wave filter element, a sufficient Out-of-band attenuation was obtained

[Brief description of the drawings]

FIG. 1 is a diagram showing an embodiment of the present invention.

FIG. 2 is a diagram showing an embodiment of the present invention.

FIG. 3 is a diagram showing a comparative example with respect to the embodiment of the present invention.

FIG. 4 is a diagram showing a comparative example with respect to the embodiment of the present invention.

[Explanation of symbols]

11, 21, 31, 41 Printed circuit board 12, 22, 32, 42 Surface acoustic wave filter element 13a, 23a, 33a, 43a Input terminal of surface acoustic wave filter element 13b, 23b, 33b, 43b Output of surface acoustic wave filter element Terminals 14a, 24a, 34a, 44a Input matching circuit 14b, 24b, 34b, 44b Output matching circuit 15, 25, 35, 45 Transmission line 16, 26, 36, 46 Parallel capacitance element 17, 27, 37, 47 Parallel Inductance element

Claims (5)

[Claims] 1. A surface acoustic wave filter device comprising: a surface acoustic wave filter element; and an impedance matching circuit connected to an input end and / or an output end of the surface acoustic wave filter element. The filter device is provided on a substrate on which a transmission line is formed, wherein the impedance matching circuit includes at least one parallel inductance element and at least one parallel capacitance element, and is provided from an input / output end of the surface acoustic wave filter element. Wherein the parallel capacitance element and the parallel inductance element are arranged and electrically connected in this order. 2. The surface acoustic wave filter device according to claim 1, wherein the parallel inductance element and the parallel capacitance element are disposed only on one side of a transmission line and are electrically connected. Surface wave filter device. 3. The surface acoustic wave filter device according to claim 1, wherein the parallel inductance element and the parallel capacitance element are respectively disposed and electrically connected to both sides of a transmission line. Surface acoustic wave filter device. 4. The surface acoustic wave filter device according to claim 1, wherein the surface acoustic wave filter device has an operating frequency of 100 MHz.
z is equal to or greater than z. 5. The surface acoustic wave filter device according to claim 1, wherein the parallel inductance element is a chip inductor, and the parallel capacitance element is a chip capacitor. Surface acoustic wave filter device.